1. Field of the Invention
The present invention relates to a dynamic pressure-type hydrodynamic bearing device, as well as a spindle motor and information device using the same.
2. Description of the Prior Art
A hydrodynamic bearing device comprises a shaft and a sleeve that supports the shaft, and a lubricant that is interposed in the gap between the two parts. With rotation of the shaft, the lubricant is gathered up by dynamic pressure-generating grooves that are formed on the shaft or sleeve, and generate pressure such that the shaft is supported within the sleeve without coming into contact therewith. As a result, when high-speed rotation is attained, ambient noise during the rotation can be alleviated.
A spindle motor equipped with such a hydrodynamic bearing device can provide the requisite rotational accuracy with an increased recording density of the medium, and can furthermore provide excellent shock resistance and quietness. Thus, it can be used in a majority of motors for application in such representative magnetic disk devices as information technology equipment and audio-visual equipment.
However, in the particular case of a spindle motor equipped with this type of hydrodynamic bearing device used in magnetic disk devices, an electrostatic charge is generated by the flow of the lubricant that is interposed between the shaft and the sleeve and the air friction of the magnetic disk due to the high-speed rotation without contact between the sleeve and the shaft of the hydrodynamic bearing device, and this electric charge will be accumulated within the device. This accumulated electric charge can be discharged between the magnetic disk connected either to the shaft or the sleeve and the record/replay head, raising a concern about record/replay failures or damage to the record/replay head.
Countermeasures against this that have been proposed include magnetic disks where organic polymers as conductivity-enhancing additives are added to the lubricant (for example, see PCT 2000-500898 Official Bulletin citation), hydrodynamic bearing spindle motors where antistatic additives are added to the lubricant (for example, see Japanese published unexamined application No. 2001-208069), hydrodynamic bearing devices having lubricants that are formulated with specific additives (for example, see Japanese published unexamined application No. 2003-171685), and the like. In this type of technology, various additives are added to the lubricant in order to increase the conductivity or antistatic effect so that either a ground is provided for the electric charge within the device or the electrostatic charge is suppressed.
In addition, in recent years, the demand has grown for magnetic disk devices that are increasingly miniaturized, more energy-conserving and progressed with operational lifetime, for decreased power consumption of motor and improvement of endurance for the spindle motor that is the main component.
For this reason, esters such as dioctyl sebacate (DOS), dioctyl azelate (DOZ), and dioctyl adipate (DOA) have been proposed for use as lubricants in hydrodynamic bearing devices. Moreover, esters obtained from neopentyl glycol and C6 to C12 monovalent fatty acids and/or their derivatives for use as lubricants in hydrodynamic bearing devices (see for example Japanese published unexamined application No. 2001-316687), the use of esters represented by the generic formula R1—COO-(AO)n—R2 as lubricants for bearings (see for example Japanese published unexamined application No. 2002-206094) have been proposed. The use of low viscosity lubricants can also result in reduced torque (in other words, low power consumption).
In addition, since the magnetic disk devices has been used extensively, and in-car equipment as represented by car navigation system is used in a wide range of temperatures relative to conventional equipment, the demand has grown for magnetic disk devices which is possible to rotate even at the low temperature.
Countermeasures against this that have been proposed include hydrodynamic bearing device where esters obtained by trimethylolpropane and at least two types of mixed acids of monovalent fatty acids having C4 to C8 are used as the lubricant (for example, see Japanese published unexamined application No. 2004-91524), hydrodynamic bearing device where esters as represented by general formula R1O-(A1O)—OC—R3-CO—(OA2)-OR2 are used as the lubricant (for example, see Japanese published unexamined application No. 2006-96849), and the like. In these types of technology, these devices will be able to work even at low temperature by using various lubricants which have low-pour points and unsolidify even at low temperature.
However, there are problems with these conventional devices as follows.
The viscosity of the lubricant will rise after adding more than a certain amount of a high molecular weight or a high viscosity compound as an additive in order to increase the conductivity or antistatic effect, which leads to the problem of increased torque in the bearing device (in other words, increasing the power consumption).
Moreover, with the objective of increasing the specific effects of the additive (conductivity from a conductivity-enhancing additive, or prevention of static charge from an antistatic additive), the heat resistance and durability of the additive itself can exert an influence on the other functions of the lubricant. Furthermore, as the additive undergoes degradation, the reduced effectiveness of the additive can cause a progressive degradation of the entire lubricant, leading to the problem that long-term reliability cannot be achieved for the device.
Further, while it is possible to reduce the torque in such conventional hydrodynamic bearing devices using the low viscosity lubricant, since the heat resistance of the lubricant is low (vapor pressure is high), the amount of evaporation will be significant when used over a long period, and it will not be possible to maintain the quantity of lubricant required for stabilized rotation of the bearing device. Consequently, there will be problems with the device having inadequate reliability and the operational lifetime will be shorter. As a countermeasure to the amount of evaporation, one can consider a method by which the above requirement is addressed by adding an excess of the lubricant. However, this approach will entail problems in that this additional amount can increase the torque and bring a higher cost, and accommodating the additional space will make miniaturization more difficult.
Moreover, with conventional hydrodynamic bearing device where the lubricant with low-pour point is used, while it is possible to work at low temperature since changes of temperature in viscosity is large (in other words, viscosity index is small), there is the problem of increasing the power consumption at low temperature. Also, especially with the hydrodynamic bearing device where the dicorboxylic acid diester base lubricant is used, pyrolysis temperature is low and stability for hydrolysis is small, which leads to the problem that long-term reliability cannot be achieved for the device under a condition such as high temperatures and humidity.